Field of the Invention
This invention is directed to novel encapsulated sub-micron sulfur particles which are suitable for utility as an electrode active material. In particular, the invention is directed to sub-micron sulfur particles which are formed in the presence of a mixed hydrophilic/hydrophobic copolymer. The resulting encapsulated sulfur sub-micron core particle is coated with a membrane of layers of self-assembling conductive polymer layers, each successive layer having a charge opposite to the previous layer. The invention is also directed to a cathode containing the membrane coated encapsulated sulfur sub-micron particles and an electrochemical cell or battery containing the cathode. The invention is further directed to a lithium-sulfur battery containing the membrane coated carbon-sulfur composite cathode.
Discussion of the Background
In an ongoing effort to develop alternative vehicle energy power sources to the combustion engine, one area of development has been a plugin electric vehicle. To date much of the effort has been directed to lithium ion batteries as a power source for such vehicles. However, to become mainstream and to compete with the combustion engine in terms of cost and driving range, a significant improvement in the energy density of lithium ion batteries is necessary. The Holy Grail of post lithium ion research is to increase energy densities by utilization of conversion cathodes with high volumetric capacities such as sulfur or oxygen in combination with a pure metal anode. As an active cathode material, elemental sulfur can provide five times higher capacity than conventionally employed materials based on transition metal oxides or phosphates.
Although studies in lithium/sulfur battery date back to the early 1960's, the system has yet become commercially significant due to limited cycle life. Various problems have hindered the practical use of this highly attractive cathode including the insulating nature of sulfur which retards its reduction and poor electrode stability owing to the solubility in the electrolyte of the polysulfides generated during reduction of sulfur (Li2Sx, x=8, 6, 5 and 4). Over the last fifty years, methods for preventing migration of polysulfides have been intensively investigated by research teams worldwide. Significant advancements have been achieved by optimizing the electrolyte composition or replacing the liquid based electrolyte with polymeric electrolytes.
One approach to solving this problem is to restrain the polysulfides generated by constraint of the sulfur into metal organic frameworks or a conductive host such as porous carbon. However, this technique offers only a partial confinement to the polysulfide species, and capacity losses observed after 100 cycles are often too great to provide useful lifetime. In other approaches to further confine the highly polar polysulfide species, the surface of the carbon has been adjusted by functionalizing with inorganic oxides or polymers with the aim of providing an exterior coating to restrict migration of polysulfides.
While there have been significant advancements in understanding the challenges of Li—S batteries, the development of ion selective membranes may be crucial to actual commercialization due to their potential to impede lithium polysulfide dissolution while permitting rapid diffusion of lithium ions.
Fu et al. reported discharge rates as high as 1C obtained with a sulfur composite containing a mixed ionic-electronic conductor (MIEC) (Fu, Y.; Manthiram, A. Enhanced Cyclability of Lithium-Sulfur Batteries by a Polymer Acid-Doped Polypyrrole Mixed Ionic-Electronic Conductor. Chem. Mater. 2012, 24, 3081-3087). The MIEC is a polymer doped polypyrrole (PPy) which is synthesized by oxidation polymerization of pyrrole monomer using ammonium peroxydisulfate as an oxidant in the presence of poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAAMPSA). The oxidized (p-doped) form of PPy is treated with an acid for removing an electron from its backbone and produces a free radical and cation (also called a polaron). The cation couples with the sulfonic acid moiety in PAAMPSA, resulting in a polyelectrolyte complex with favorable ionic and electronic properties. The S-MIEC composite cathode showed improved electrochemical stability. It is claimed that the polyelectrolyte complex facilitates ion and electron transfer while capturing intermediate polysulfides via ion exchange. A capacity of 500 mAh/g S (45% sulfur in cathode) can be retained after 50 cycles at a high rate of 1C.
A different type of polymeric complex was reported by Wang et al (Wang, L.; Wang, D.; Zhang, F.; Jin, J. Interface Chemistry Guided Long-Cycle-Life Li—S Battery. Nano Lett. 2013, 13, 4206-4211). A polydopamine (PD)/poly(acrylic acid) (PAA) complex held together by a cross link reaction which forms a robust network containing the GO/S components via rigid covalent bonds was constructed. An initial capacity of 1350 mAh/g was reported for the first cycle at a low discharge rate of approximately C/3 (60% sulfur cathode content). A 45% capacity fade was observed over 500 cycles. The report claims the rigidity of the framework is advantageous for inhibition of polysulfide loss but no remarks are given on the impact it has on volume change mechanics between charge and discharge.
A concept similar to that used by Fu et al is the utilization of an ionic shield for polysulfides. Sulfonate-end capped (SO3−) perfluoroalkyl ether groups of Nafion coated Celgard 2400 allow ion hopping of positively charged Li+ but inhibits hopping of negative polysulfides due to coulombic interactions (Huang, J.-Q.; Zhang, Q.; Peng, H.-J.; Liu, X.-Y.; Qian, W.-Z.; Wei, F. Ionic Shield for Polysulfides towards Highly-Stable Lithium-sulfur Batteries. Energy Environ. Sci. 2013, 7, 347-353). This cation “permselective membrane” is claimed to act as an electrostatic shield for polysulfide ions and retained about 425 mAh/g capacity after 500 cycles at a high rate of 1C (50% sulfur cathode content). Two oxidation waves were reported in the cyclic voltammograms corresponding to a stepwise transformation between S8 and Li2S4.
The concept of reducing the mobility of polysulfides based on their ionic properties utilized by Fu et al in their utilization of MEIC polyelectrolyte complexes was also the foundation for the approach reported by Bucur et al (Bucur, C. B.; Muldoon, J.; Lita, A.; Schlenoff, J. B.; Ghostine, R. A.; Dietz, S.; Allred, G. Ultrathin Tunable Ion Conducting Nanomembranes for Encapsulation of Sulfur Cathodes. Energy Environ. Sci. 2013, 6, 3286-3290), where carbon/sulfur composites were encapsulated with ultrathin (5 nm) polyelectrolyte multilayers (PEML) composed of three to five layers of sequentially adsorbed polyions having opposing charges. Poly(diallyalmmonium chloride) (PDAD) and poly(styrene sulfonate) (PSS) were employed to form the PEML's. Such PEMLs have been reported to benefit from highly conformal coverage and precise control over their assembly. They have also been reported to be flexible and self-healing. These properties are ideal for sulfur cathode applications. They possess an amorphous, interpenetrated bulk structure consisting of a majority of intrinsic sites between two oppositely charged polymers and a minority of extrinsic sites between solution ions and polyelectrolyte backbone charges. Diffusion through the membrane is governed by the number of extrinsic sites which permits hopping of ions. An ion with a single charge (lithium salts) will hop more freely than a divalent ion (polysulfides). It is interesting to note that the content of extrinsic compensation can be adjusted by the ionic strength of the solution in which assembly of the membrane occurs. Membranes assembled from high ionic strength solutions result in poor capacity retention because the higher number of extrinsic sites promotes the diffusion of both the lithium and polysulfide ions. A capacity of approximately 600 mAh/g S was reported after 300 cycles at a high rate of 1C and after 100 cycles at an ideal rate of 5C (45% sulfur cathode content). Facile oxidation (recharging) kinetic behavior was displayed by the two waves visible in the cyclic voltammogram right below and above 2.3V.
While various and intricate carbon/sulfur composites have been reported to increase the electronic conductivity of sulfur, it has been the use of polymers as interface modifiers, composites or coatings which have drastically improved the cyclability of sulfur cathodes by presumably trapping or limiting the diffusion of polysulfides. In addition, polymers can provide an elastic framework for sulfur which can accommodate the ˜20% volume expansion reported between the charged (elemental sulfur, density ˜2.0 g/cm3) and discharged species (Li2S, density 1.66 g/cm3). Attempts to provide rigid encapsulation to sulfur (carbon or oxide based) could not provide a stable barrier to polysulfide diffusion due to rupturing during cycling.
In addition, the use of polymers as encapsulating membranes for sulfur particles provides opportunity for reducing the high polymer contents associated with sulfur/polymer composites and thus the overall capacities based on total cathode mass may be increased. Ideally, while providing a barrier to migration of the bulkier polysulfides the encapsulating membrane does not impede electronic or ionic conduction of lithium ions. For optimum performance the membrane must be comparatively thin, conformal to the encapsulating substrate and flexible to accommodate the volume changes of the sulfur core between charge and discharge.
In order to extend the lifetime beyond that offered by carbon/sulfur composites, a team comprising the present inventors encapsulated carbon-sulfur composites with tunable, self-assembled nano-membranes which restrict the diffusion of polysulfides while allowing for diffusion of the lithium ions. (Bucur, C. B.; Muldoon, J.; Lita, A.; Schlenoff, J. B.; Ghostine, R. A.; Dietz, S.; Allred, G. Ultrathin Tunable Ion Conducting Nanomembranes for Encapsulation of Sulfur Cathodes. Energy Environ. Sci. 2013, 6, 3286-3290) Batteries with cathodes constructed with these encapsulated sulfur composites operate for more than 300 cycles with less than 15% capacity loss (capacities above 600 mAh/g) at a high rate of 1C.
While the self-assembled nano-membranes inhibit polysulfide dissolution, the final cathode structure contains a low sulfur loading of approximately 50% due to the use of the melt-infusion of sulfur into high surface area carbon. The resulting carbon/sulfur composite has a high content of carbon. In order to obtain a Li/S battery with energy densities more than two times higher than Li-ion batteries, sulfur loading greater than 75% by weight is required.
Much effort directed to increasing lithium-sulfur battery capacity has been reported.
Skotheim et al. (US 2014/0205912) describe lithium batteries having a lithium anode which is protected by a multi-layer coating structure. Each of the films of the multi-layer structure allows passage of lithium ions, but act as a barrier for other cell components to reach the lithium metal surface. Electrochemical cells and batteries containing the multi-coated anode are described and cathodes constructed of a mixture of elemental sulfur, a conductive carbon material, and a binder. Conductive polymer materials are disclosed as possible carbon constituents. However, as indicated in Example 7, the materials are mixed or blended and applied to an electrode substrate. Sub-micron sulfur particles encapsulated in conductive polymer multilayer systems as active cathode components are neither disclosed nor suggested.
Pyun et al. (U.S. 2014/0199592) describes sulfur composites which are copolymers of elemental sulfur and comonomers selected from ethylenically unsaturated monomers, epoxide monomers and thiirane monomers. The potential value of a lithium-sulfur battery is recognized and the deficiencies of conventional elemental sulfur based cathodes are described. Thus this reference addresses the stability problem by incorporation of copolymer having a high content of copolymerized sulfur and one or more of the previously listed comonomers. A comonomer composite containing conductive filler is described wherein the conductive filler is either present during the polymerization or is physically blended with the formed copolymer.
Archer et al. (U.S. 2013/0330619) describes a cathode active material for a lithium-sulfur battery that is a mesoporous carbon containing infused sulfur. The mesoporous carbon matrix is obtained by forming a highly porous metal oxide template, fusing the metal oxide template with a carbonaceous material such as pitch and sintering. The metal oxide is then etched from the sintered mass leaving a mesoporous carbon into which gaseous sulfur is infused. The material is then milled to small size. Archer does not disclose or suggest sub-micron sulfur particles encapsulated in conductive polymer multilayer systems.
Wang (U.S. 2013/0171355) describes a sulfur graphene composite material which can be employed as an active ingredient of a cathode in a lithium-sulfur battery. The composite is prepared by mixing a dispersion of graphene and a solution of sulfur, precipitating the sulfur onto the graphene, removing the solvents and treating the residue to form an active material.
Li et al. (U.S. 2013/0065128) describes lithium-sulfur batteries with cathode active materials being hollow nanostructures (tubes, fibers, spheres) containing elemental sulfur and/or an active sulfur compound. According to Li, the sulfur does not occupy the entire volume of the hollow space, such that allowance is made for expansion and contraction of the sulfur material during the charging and discharging phases. The hollow nanostructures are first formed and then the sulfur/sulfur compound infused. Sub-micron sulfur particles encapsulated in conductive polymer multilayer systems as active cathode components are neither disclosed nor suggested in the description of this reference.
Zhamu et al. (U.S. 2011/0165466) describes lithium-sulfur batteries wherein the cathode is constructed of a nano-filament web of carbon nano fibers, graphite nanofibers, carbon nanotubes, etc. Upon construction of the cell, lithium sulfide and/or lithium polysulfide is deposited on the nano-filaments and upon charge these materials convert to elemental sulfur on the cathode and lithium metal at the anode. Zhamu does not disclose or suggest cathode architecture of sub-micron sulfur particles encapsulated in conductive polymer multilayer systems.
Naoi et al. (U.S. 2007/0287060) describes a cathode active sulfur composite of elemental sulfur and/or a compound having a sulfur-sulfur bond with microparticles of a conductive material such as carbon black (Ketjen Black). A mechanical composite of the sulfur and carbon black is first formed. The thus formed composite is heated above the melting point such that the conductive microparticles are infused into the sulfur melt which is then stressed into a fibrous form. This is cooled and pulverized and a thin uniform coating of the conductive microparticles applied to the surface of the pulverized particles. The thus formed composite material is the active component of a cathode for a lithium-sulfur battery.
Choi et al. (U.S. 2005/0053718) describes a cathode material for a lithium-sulfur battery wherein particles of elemental sulfur are coated with an inorganic salt such as an alkali metal alkoxide, a borate salt, a metal oxide or a silicate. The coating is applied to the sulfur core from solution with drying.
Nazri et al. (U.S. Pat. No. 8,663,840) describes a cathode active material for a lithium-sulfur battery that consists of carbon nanotubes which contain sulfur or a sulfur compound within the hollow interior of the nanotube. The nanotubes are first formed and then the sulfur component placed in the hollow area by melt infusion, sublimation or solution filling followed by evaporation of the solvent. The exterior of the filled nanotubes is then cleaned of sulfur residue and a cathode constructed with the composite nanotubes.
Wang et al. (CN103474633) (Abstract only) describes a complex composite and cathode structure for a lithium-sulfur battery containing the composite. The composite is comprised of a core of nano-carbon particles which is over coated with elemental sulfur or a polysulfide mixture. Alternatively, the nanoparticles are surface bonded with a polymeric cross-linking system and the sulfur material is incorporated in that cross-linking matrix. This composite is coated onto a carbon nanoparticle sulfur core-shell material. The mixture is coated to a substrate to form a positive electrode which is functional for a lithium-sulfur battery.
Li et al. (Proc. Natl. Acad. Sci., 2013, 2) describes a core-shell nanoparticle having a sulfur core prepared by insitu precipitation of the sulfur in the presence of a polymer. A core shell nanoparticle coated with poly(3,4-ethylenedioxythiophene) (PEDOT) is described.
Manthiram et al. (Chem. Rev. 2014, Special Issue: 2014 Batteries: DOI: 10, 1021/cr500062v) provides a review of progress in sulfur based materials for lithium-sulfur batteries. A carbon-sulfur composite coated with PEDOT:PSS is described.
In order to provide lithium-sulfur batteries having significantly increased energy density, high capacity cathode materials are necessary.
Thus, an object of the present invention is to provide a sulfur composition which is suitable for utility as an electrode active material for a battery having high capacity and high cycle lifetime.
A second object of the invention is to provide a cathode containing sulfur as an active material which is suitable for utility in a battery having high capacity and high cycle lifetime.
A third object of the invention is to provide a lithium-sulfur battery which has sufficient capacity and lifetime to be a viable energy source for a vehicle.